1. Field of the Invention
The invention is related generally to the field of drilling boreholes through the earth. More specifically, the invention relates to methods for selecting and designing drill bits for drilling such boreholes so that petroleum fluid extraction from subsurface formations is optimized.
2. Description of the Related Art
In drilling oil and gas wells, drill bits are used to drill a borehole through earth formations. The formations include “overburden”, which generally is all the formation layers above a producing formation. The producing formation includes a deposit of oil and/or gas in pore spaces therein. Typically, the vast majority of the length of the hole, in a conventional vertical or near vertical well is through the overburden. The drill bits are usually selected based on expected drilling performance criteria, such as maximum rate of penetration, maximum footage drilled per bit, minimum cost to drill to the target depth, etc. The focus is to drill the borehole as fast as possible and with as few “trips” (operations in which an entire drilling tool assembly is removed from the wellbore) as possible.
However, once the borehole has been drilled to the top of the producing zone, various production performance criteria come into play that are not as important when drilling through the overburden. The portion of the borehole through the production zone is the most critical because this is the portion through which the oil and/or gas will ultimately be produced. If this portion of the borehole is not drilled and conditioned properly, the economics of the well and the entire reservoir may be substantially impaired. For this reason, borehole operators are willing to sacrifice drilling performance to some extent to ensure that the condition of the borehole through the production zone is optimized.
For example, it is desirable for the permeability of the earth formations proximate the wall of the borehole not be diminished. Typically, however, solids from a drilling fluid (“mud”) used to drill the borehole, and/or formation drill cuttings become embedded into the formation near the borehole wall and create a “skin” that can impede the flow of oil and/or gas into the borehole. If the cuttings are large and remain large as they travel up the bore hole they are less likely to embed in the formation near the borehole wall and the degree of skin formation is lessened. Smaller cuttings, that have a higher propensity for contributing to skin, may be caused by a particular bit that creates small cuttings in the particular producing formation. Often the hydraulics and/or design of the drill bit cause initially large cuttings to be reground by the bit during drilling, thereby reducing the cuttings to a size that increases skin formation.
In view of the more critical nature of the borehole through the producing zone and the differing parameters that are sought to be optimized, a need exists for a drilling method that changes the bit selection criteria once the production zone has been reached.